Earnings and Dividends Per Share (Summary Of Calculation Of Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to common stockholders	$ 37,798	$ 39,092	$ 38,245	$ 44,230	$ 41,188	$ 33,032	$ 44,595	$ 32,725	$ 159,365	$ 151,540	$ 148,862
BASIC:
Weighted Average Number of Shares Outstanding, Basic	41,532,130	41,108,699	40,982,244	39,953,804	39,847,860	39,283,919	38,520,221	38,401,647	40,894,219	39,013,412	37,604,594
Net income per common share attributable to common stockholders	$ 0.91	$ 0.95	$ 0.93	$ 1.11	$ 1.03	$ 0.84	$ 1.16	$ 0.85	$ 3.90	$ 3.88	$ 3.96
DILUTED:
Weighted average common shares outstanding, diluted									40,894,219	39,013,412	37,604,594
Stock options, diluted									67,703	52,497	34,842
Convertible subordinated debentures									189,531	89,471	4,735
Average dilutive common shares outstanding	41,803,615	41,448,263	41,245,173	40,108,762	39,993,445	39,651,900	38,561,384	38,414,791	41,151,453	39,155,380	37,644,171
Net income per common share attributable to common stockholders	$ 0.90	$ 0.94	$ 0.93	$ 1.10	$ 1.03	$ 0.83	$ 1.16	$ 0.85	$ 3.87	$ 3.87	$ 3.95
Incremental shares excluded since anti-dilutive:
Stock options with an exercise price in excess of the market price for our common shares									573	6,366	51,603
Common Stock, Dividends, Per Share, Declared, Regular									$ 3.80	$ 3.60	$ 3.40